In PET with time-of-flight (TOF) capability, reconstruction algorithms rely on precise timing calibration for each line-of-response (LOR). Current calibration methods use a positron emission point source, a cylinder source, or patient data to ascertain scintillator crystal timing. However, these sources are not advantageous for timing calibration of a PET system. Point source is small and no direct coincident event counts are available for many valid LORs. Cylindrical source is so large that along each LOR the source is distributed in a large range.
The cylinder calibration source is placed in the center of the scanner. There are some LORs that do not intersect the phantom, hence cannot be calibrated directly. In this case the actual timing difference between the scintillation crystals that define the non-intersecting LOR can only be indirectly derived. The implicit assumption behind the indirect derivation is either the delay at a crystal is independent of gamma incidence angle, or the delay at each crystal is always the same. However, larger, i.e. shallower, incidence angles may cause extra delay due to decreased average depth of interaction (DOI) of gamma photons in the scintillation crystal. The visible scintillation photons travel slower than gamma photons in the scintillation crystal which leads to timing differences attributable to the proportion of the crystal length traversed by the gamma photons. To calibrate a ring-type scanner, each LOR from the calibration source strikes the crystals at both ends the same incidence angle. Therefore it is assumed the extra delay caused by increased incidence angle is the same.
The activity in the cylindrical source is distributed in a range along the LOR, so determining the crystal timing for the LOR to the same precision as a point source requires a larger number of counts than when the activity concentrated at a single spot along the LOR. Additionally, the chance of encountering Compton scattering of the coincidence photons increases with the volume of the source. Inclusion of scattered photons in the measurement further reduces the crystal timing precision.